An electric motor (hereinafter, referred to as only a motor) has been widely used as a means generating power of electric/electronic devices in various industrial fields, and efforts to implement environmentally friendly products and reduce power consumption as indices of competitiveness of each manufacturer have been made.
In the related art, a motor driving device for driving the motor has used a boost converter.
The motor driving device using the boost converter according to the related art described above constantly controls a direct current (DC)-link voltage output by the boost converter in an entire load region, and thus, a system design and control are simple. However, a large torque ripple and speed ripple appear in a low speed section, and back electromotive force (EMF) of the motor is large in a high speed section, such that a control of a predetermined speed or more is impossible when a weak-field control is not applied.
Another motor driving device for driving the motor according to the related art has used a series-type buck-boost converter.
The motor driving device using the buck-boost converter may control a switching operation of a switch included in the buck-boost converter depending on a rotation speed of the motor to vary a DC-link voltage, and control the varied DC-link voltage and a switching pattern of an inverter to control the rotation speed of the motor. The buck-boost converter capable of stepping up and stepping down a voltage may be applied to a DC-link voltage variable inverter system requiring a wide load range. That is, in a section in which the motor is driven at a low speed, the motor driving device using the buck-boost converter may step down the DC-link voltage to improve driving performance of the motor, and in a section in which the motor rotates at a high speed, the motor driving device using the buck-boost converter may step up the DC-link voltage to a voltage higher than back electromotive force of the motor to control the motor without using an additional weak-field operation algorithm in a weak-field region.
Meanwhile, the motor driving device using the buck-boost converter according to the related art has used a boost control and a buck-boost synchronization control as two control manners of converting an input voltage by the buck-boost converter. The boost control and the buck-boost synchronization control, which are the two control manners, are determined as follows. The buck-boost synchronization control is used in the case of intending to vary the DC-link voltage in a range equal to or smaller than a maximum boundary of an input voltage input to the buck-boost converter, and the boost control is used in the case of intending to vary the DC-link voltage in a range equal to or larger than the maximum boundary of the input voltage.
In more detail, in the boost control, a boost control of stepping up the input voltage by maintaining a switch stepping up the input voltage in a switch-on state and switching another switch is performed. In addition, the input voltage is stepped down depending on a duty ratio using a pulse width modulation (PWM) control signal simultaneously switching two switches.
When the DC-link voltage input to the inverter is stepped down in a low load region in which the motor is driven at a low speed, the respective switching voltages of switches included in the inverter are stepped down, such that switching loss is reduced, and thus, an inverting efficiency of the inverter itself is increased, but in the case of the buck-boost synchronization control manner, the two switches are simultaneously switched on/off, such that switching loss is increased. In addition, since an input current is always discontinuous, a power factor and total harmonic distortion (THD) performance are low. As a result, in a light load region in which the input voltage is stepped down through the buck-boost converter, an efficiency of the inverter and the motor may be increased, but a system efficiency of the entire motor driving device is not improved.